Digital transmission systems employing, for example, differential PCM encoding and decoding of signals, typically use adaptive quantizers and adaptive predictors. In such arrangements, a plurality of previously generated or received samples is used to generate a predicted value. In the encoder, the predicted value is algebraically subtracted from a received signal to generate a differential error signal which is quantized into a differential PCM signal sample for transmission. In the decoder, the predicted value is algebraically summed with a quantized version of a received differential error signal to reconstruct the original signal that was supplied to the encoder. In such arrangements, it is important that the encoder and decoder experience the same quantizer and predictor states. Otherwise, the reconstructed signal being generated in the decoder is unduly distorted relative to the signal being supplied to the encoder.
The encoder and decoder may experience different quantizer and/or predictor states when gaps occur in the transmission because of switching or the like in the transmission channel. This did not occur in prior transmission arrangements, because the encoder and decoder were operated continuously.
In packet transmission systems, an additional factor is so-called delay modulation of gaps between packets. This delay modulation is cause by variable delay in the packet system. Consequently, the gap intervals between packets at the decoder are not the same as at the encoder. The delay modulation will also cause distortion in the reconstructed signal which is undesirable.
Additionally, if bit errors occur in transmission, the decoder receives incorrect code words and operates erroneously for some interval of time because of the use of the plurality of past samples in the adaptive quantizer and/or predictor. This also causes undesirable distortion in the reconstructed signal.